The present invention relates to contact lenses, and in particular to an improved means by which the angular orientation of the lens upon the eye of a wearer can be maintained.
Contact lenses, i.e. transparent lenses worn directly upon the surface of the eye, have long been known in opthalmology. Although various types of contact lenses are presently known and used, they are generally catergorized as "hard" or "soft" lenses. Of these two types, the so-called "hard" contact lenses are presently better known and have been in use longer. Such lenses may be formed of numerous substances, but are commonly formed of crosslinked polymethylmethacrylate and other polymeric materials. The so-called "hard" contact lenses are substantially smaller than the cornea of the eye.
The newer, "soft" contact lenses may be formed of many materials including hydroxyethylmethylmethacrylate, metallo-organic substances, silicone rubbers, and various other polymers such as disclosed in U.S. Pat. No. 3,503,942 and 3,639,524--Seiderman et al. Many of the soft contact lenses currently in use are hydrophilic, that is, they absorb water and, in fact, the water becomes an integral part of their structure. All of the so-called "soft" contact lenses exhibit much greater flexibility than "hard" contact lenses and can undergo considerable distortion and subsequently return to their original configuration. Hard contact lenses, on the other hand, are comparatively rigid. Another distinction between the "hard" and "soft" types of lenses is that the soft lenses are commonly much larger, ordinarily being of sufficient diameter to cover the cornea of a wearer's eye.
Spherical contact lenses of both hard and soft types are in the majority. The optical power of the lens does not vary either radially or circumferentially about the optic axis of such lenses, so that they may be rotated at any angle without changing the optical effect upon a wearer.
As is well known to those skilled in the art, certain types of defects in the eye can only be corrected by lenses which are not spherical. For instance, to effectively correct for astigmatism an ophthalmic lens must exhibit a cylindrical characteristic so that rays of light are preferentially directed in certain planes. Also, presbyopia is commonly corrected by means of "bifocal" ophthalmic lenses which exhibit different optical properties at different portions thereof.
Providing cylindrical, bifocal or multifocal characteristics in a lens causes systematic deviations from spherical symmetry. This may be done to correct for conditions in the optical elements of the eye, i.e. the cornea and the crystalline lens, in that they do not comprise spherical surfaces. The latter condition, termed astigmatism, can be corrected with lenses by complementing the aspherical nature of cornea and natural lens in the eye.
One form of lens used to correct the astigmatic eye is a lens which has the meridians of least and greatest curvature located at right angles to one another on the same surface. This is designated as a toroidal surface and a lens having such a surface is referred to as a toric lens.
Various forms of contact lenses may be used to correct for presbyopia. All such lenses exhibit different optical properties in at least two different portions of the lens. Many such lenses are not spherical and must, therefore, be maintained at a specific orientation with respect to the meridians of the eye.
When spectacles are worn, proper orientation of the meridians of a toric lens with respect to the eye is not a problem inasmuch as the rigid frame of the spectacles holds the lens in the proper orientation. With contact lenses, however, there is no such rigid mechanism or frame to hold the lens in place. Vertical and lateral location of the lens is generally maintained by fitting the curvature of the cornea. An optimum fitted spherical contact lens will rotate during wear without any loss in visual acuity. A toric or otherwise spherically asymmetrical lens, to be effective, must not rotate.
Angular orientation of a well-fitted contact lens is difficult to achieve. Various approaches have been devised in an attempt to cause contact lenses to maintain a preferred orientation upon the eye. Such approaches have generally comprised modifications to the physical form of lens structure. These approaches often place undue constraints upon the shape of the lenses, and in some cases may restrict the prescription of the lens. Therefore, efforts to achieve preferential orientation of contact lenses have not been universally successful. In fact, often configurations which provide preferential orientation and good visual acuity for some individuals will not do so for others.
It will therefore be seen that it is highly desirable to provide a universal means and method for causing a spherically asymmetrical contact lens to consistently maintain a given preferential angular orientation on the eye of a wearer.
It is therefore an object of the present invention to provide an improved contact lens which achieves and maintains a preferred angular orientation upon the cornea of the eye.
Another object of the present invention is to provide a method for achieving a predetermined angular orientation of a spherically asymmetrical contact lens upon the cornea of the eye.
Yet another object is to provide a contact lens which consistently achieves a predetermined angular orientation.
Still another object is to provide a means for achieving a given angular orientation of a contact lens at a predetermined angle, without regard to the specific curvature of the lens.